This invention relates to the field of devices for determining the characteristics of acoustic transducers. More particularly it concerns an apparatus for performing an in situ determination of the operational parameters of an elongate hose-like acoustic array. In still greater particularity the improved apparatus for enabling the determination of operational characteristics employs a movable cart-like structure that positions a compact, low-frequency source at desired test points along the array's length, or, alternately, the array is pulled through the cart-like structure at a uniform speed as a sound source incorporated in the cart ensonifies the passing hydrophone elements.
Increasingly, the gathering of acoustical data from the oceans is becoming more significant in terms of research and from military considerations. Long line arrays made up of acoustic channels, each monitoring the pressure field through one or generally a number of hydrophone elements arranged in series along the array over a length extending hundreds of feet are able to provide data for highly sophisticated processors. Usually such hydrophone arrays are coupled to a towing vessel or are otherwise vertically or horizontally disposed in the open ocean so that the individual hydrophone elements can provide indications of the acoustic happenings about them. Properly phased and controlled, the hydrophones create steerable beams for determining bearings from acoustic sources or can be otherwise controlled to provide desired responses.
However, the meaningful processing of the received information requires that the operating characteristics of the individual hydrophones or groups of hydrophones forming the front end of acoustic channels in the elongate array be known or at least predictable. As a consequence the calibration must occur frequenly enough to ensure that the array undetected degredations to not occur, particularly since the array operates in the harsh marine environment which oftentimes adversely affects everything near it.
Over the years a variety of techniques have been developed for calibrating individual hydrophone elements of a large array. One approach distantly projects calibration or test signals of selected frequencies and magnitudes on the arry so that it is received nearly simultaneously by every hydrophone element. In a popular calibration technique the sections of an elongate array are coiled on a cylindrical form having an acoustic projector at the center and a standard hydrophone next to the array. This assembly is lowered into a calibration pool for conducting the acoustic tests.
One somewhat more acceptable calibration apparatus and method are disclosed by George O. Pickens in U.S. Patent and Trademark Office U.S. Pat. No. 4,205,394. This device encloses coiled sections of the array with a reference hydrophone in a box and creates pressure fields by means of a group of loudspeakers operating in unison and serving as large pistons. While this greatly reduced the cost and risk of damage in array calibration, it failed to make the measurements under operational temperature and pressure envinronments. Largely, because of time constraints and support equipments such a calibration takes place at shore installations periodically between cruises. In the meantime a lot of invalid data would be collected as the hydrophone characteristics change during an actual deployment of the array at sea.
Yet another calibration approach indirectly determines the sensitivity of individual hydrophones by monitoring an acoustic source which is not located near or focused at any individual hydrophone. First the array is deployed and the distance source actuated. The multipath effect quite expectedly interferes with sensitivity measurements. Calibration is compromised and the sensitivity of the individual hydrophones that make up a towed array during operational conditions is indeterminable.
The nearest thing for appraising the condition of a towed array under working conditions is disclosed by George O. Pickens in his U.S. Patent and Trademark Office Ser. No. 126,589. His invention entitled "Towed Array Condition Appraisal System" causes a cart-like neutrally buoyant structure to release its hold on the forward end of an array. The structure essentially stops in the water as the array pulls through an incorporated projector. While this is happening, one of the several functions of this device is to ensonify the passing hydrophones with one or more acoustic signals of chosen frequencies and levels. Meanwhile on-board ship the resultant voltages from the acoustic channels are suitably recorded so that the relative and absolute sensitivities of the individual elements and combined groups can be determined. The restrictions on bulk and weight compounded by the great hydrostatic pressures at the desired operating depths made it difficult or impractical to achieve the necessary sound levels with ceramic projectors at frequencies below 100 Hz. A mechanical projector employing a motor-driven piston seemed to be the answer but the problem was to provide the backing compliance for the piston. In other applications this has been accomplished in two manners, both of which are troubled by great depths and tend to be inefficient. One backs the volume behind the piston with compressed air at or nearly equal to the hydrostatic pressure. The other incorporates a compliant structure which both withstands the hydrostatic pressure and complies with the volumetric displacement of the piston. With the increasing interest in monitoring and analyzing low frequency signals it has become apparent that the calibration apparatus of this system could be improved upon to provide for low frequency calibration.
Thus, there is a continuing need in the state-of-the-art for an improved apparatus having a low-frequency calibrating capability for a long line array of spaced acoustic elements.